1. Field of the Invention
The present invention relates to a compensating circuit for a mixer stage, wherein the mixer stage has an input stage to which an input signal for mixing can be applied and has, following the input stage, a switching stage for mixing a differential signal—obtained from the input stage as a function of the input signal—with an oscillator signal, wherein the input of the compensating circuit can be connected to an input signal terminal of the input stage and the output of the compensating circuit can be connected to an input signal terminal of the switching circuit at which the differential input signal obtained by the input stage is present, and wherein a compensating signal obtained by a compensating stage of the compensating circuit, in particular for compensating intermodulation products, can be produced by the compensating circuit at its output. The invention further relates to a mixer stage with a compensating circuit.
2. Description of the Background Art
Compensating circuits of the aforementioned type are known and are used in mixer stages that operate on the basis of the principle of the Gilbert cell, for example, in order to compensate for unwanted intermodulation products, in particular third-order intermodulation products, which are also called IM3 intermodulation products.
Such a mixer stage, which has associated with it a compensating circuit containing a differential stage, is known from the document “A+10-dBm IIP3 SiGe Mixer With IM3 Cancellation Technique” by Shoji Otaka et al, IEEE Journal of Solid-State Circuits, Vol. 39, No. 12, December 2004, p 2333. The described differential stage produces an IM3 signal that is added with a 180° phase shift to a differential input signal for the switching stage in order to compensate for the relevant intermodulation product at the input of the switching stage. The IM3 signal produced by the prior art compensating circuit is adjusted with regard to its amplitude as a function of a bias current for the relevant differential stage and as a function of a resistor used to implement negative feedback.
In addition to the systems described, mixers that have a compensating circuit with its own mixer stage, and that have a correspondingly complex structure, are also known.